Examples of prior art methods and arrangements for introducing fiber suspensions or pulps from a pulp storage tower or reaction vessel to a washing or filtering equipment will be discussed in the following.
A good example of the problem areas in pulp and paper industry are various washing, filtering and/or thickening stages, or washing, filtering and/or thickening means between the digestion of wood chips in chemical pulping or other wood treatment in mechanical and chemi-mechanical pulping and the web formation at a pulp or paper mill. Such means are needed, for instance in so called brown stock washing and in various washing stages when delignifying and bleaching the pulp. In pulp and paper making industry various different washing, filtering or thickening means are used. Due to the wide variety of the equipment the requirements the apparatus sets for the pulp or fiber suspension are variable, too. For instance, the inlet consistency percentage of such apparatus varies from about 5 up to about 12.
However, the apparatus, or rather the process has one requirement that is valid irrespective of the consistency of the fiber suspension, i.e. the gas or air content of the fiber suspension or pulp should always be as low as possible, as the more gas the pulp contains the more gas enters the filtrate, and the more foam tends to form in the filtrate when treating the filtrate impeding the pumping and other treatment of the filtrate. However, a fact is that the amount of gas in the pulp or fiber suspension increase when the consistency increases. Thereby the problems the gas causes are increased lately as the consistency of the pulp to be treated has been raised for reducing the water consumption. In other words, the gas content of the fiber suspension has simultaneously increased.
Another requirement the various washing, filtering and/or thickening apparatus has, is a uniform feed of fiber suspension to the washing, filtering and/or thickening apparatus. Usually the washing, filtering and/or thickening apparatus have such a wide working width that the fiber suspension is introduced in the apparatus via more than one inlet, whereby the flow in the several inlets should be as equal and uniform as possible. In other words, the volume flows via the inlets should be equal. This is not an easy task as one has to use a flow divider, which is most often a static device having one inlet and as many outlets as required. The outlets are normally provided with valves for controlling the volume flow of pulp via the outlets. Due to its nature such a flow divider works the more reliably the lower the consistency is and vice versa. For the above reason prior art knows flow dividers that are provided with a rotor for facilitating the division of thick fiber suspension to the outlets.
A third requirement the washing, filtering and/or thickening apparatus has, is a certain apparatus-specific optimal inlet pressure. In other words, each apparatus is designed for a certain inlet pressure to ensure that the division of pulp or fiber suspension on the filtering surfaces of the apparatus results in a layer of pulp having optimal properties in view of the operations the apparatus is designed for, i.e. washing or filtering.